Abstract A central problem in immunology is how the immune system launches robust immunity against invading pathogens, while maintaining tolerance to self. This problem assumes a particular significance in the intestine because of the trillions of commensal microorganisms and food antigens that confront the intestinal immune system every day. Recent advances suggest that dendritic cells (DCs) play a fundamental role in maintaining the balance between immunity and tolerance. We now know that there are multiple subpopulations of DCs that differentially regulate the immune response, and that these subsets display tremendous functional plasticity in response to instructive signals from microbes and microenvironments. In this context, our recent work suggests that DCs and macrophages in the lamina propria of the intestine differentially regulate Th17 versus T regulatory responses, and that a dynamic interplay between DCs and macrophages maintains a balance between immunity and tolerance. However, there are multiple subpopulations of DCs in the lamina propria. This raises several fundamental questions: (i) What roles do the distinct DC subsets play in regulating immunity versus tolerance? (ii) Are the functions of these cells fixed, or are they plastic? (iii) To what extent do commensals shape the function of DCs in the intestine? (iv) How can oral vaccines target intestinal DCs to stimulate optimally effective mucosal immunity? The present grant will address these questions in the following specific aims: Aim 1: To determine whether distinct subsets of lamina propria DCs and macrophages differentially bias the class of innate and adaptive immune responses Aim 2: To determine whether commensal bacterial flora influence the functions of lamina propria DCs and macrophages, via TLR signaling or non-TLR signaling Aim 3 : To determine the innate reponses of lamina propria DCs and macrophages to oral administration of adjuvants or vaccines, and the effects of such responses on the adaptive immune response The successful completion of these aims will provide fundamental mechanistic insights into how the immune system maintains a balance between immunity and tolerance, and provide new strategies for modulating mucosal immunity to control autoimmunity, or to protect against oral infections.